Later that day, Health and Human Services Secretary Tom Price backed off from the need for an emergency declaration.

"We believe that, at this point, that the resources that we need, or the focus that we need to bring to bear to the opioid crisis can be addressed without the declaration of an emergency," he said, adding that the option was still on the table.

On Thursday, the president availed himself of that option.

"The opioid crisis is an emergency, and I'm saying officially right now: It is an emergency," Trump said at an impromptu press briefing at his golf club in Bedminster, N.J.

The White House followed up with a press release saying the president "has instructed his Administration to use all appropriate emergency and other authorities to respond to the crisis caused by the opioid epidemic."

So while the president has announced an emergency, he and his administration haven't formally declared one – a process that comes with specific legal authority and brings specific sets of powers and access to money.

You can see a series of formal public health emergency declarations here.

The president could use authority under the Stafford Act to declare an emergency. That would open up resources that are usually reserved for natural disasters like hurricanes or floods, including FEMA's disaster relief fund, which had about $1.5 billion available as of July.

2. Public health workers could be redeployed.

The president could ask HHS Secretary Price to declare an emergency under the Public Health Service Act. Unlike FEMA, HHS doesn't have a standing emergency fund (although during last year's Zika virus scare, many people urged that one be established), but money could be freed up. Right now, public health workers and researchers are working on projects defined by grants from HHS. If Price were to declare an emergency, those workers could be redeployed temporarily, from working on AIDS outreach for example, to work on substance abuse issues.

3. Access to medication-assisted treatment could get a boost.

In a public health emergency, the HHS secretary could make it easier to get medications to counter addiction. For example, Price could allow "standing orders" for the drug naloxone, used to reverse overdoses, which would allow certain populations to get the medication without a prescription. He could also waive the restrictions on doctors who want to administer methadone or buprenorphine to patients with opioid addictions. Those medications currently require a prescribing doctor to have special certification, and there are limits to the number of people doctors can treat.

4. Medicaid could pay for more treatment.

A public health emergency would also allow HHS to waive certain regulations. One major target could be a rule that restricts where Medicaid patients can get inpatient drug treatment. If HHS waived that rule, then Medicaid beneficiaries might find it easier to get treatment.

5. Congress could appropriate money.

Congress doesn't need an emergency declaration to appropriate money for more drug treatment and intervention. But if the Trump administration were to issue such an order, it would put a lot of pressure on Congress to back it up with money. Last year, lawmakers passed a law to address addiction issues but refused to include $1.1 billion that President Barack Obama requested to expand treatment programs.

6. States could request aid.

Six states have already declared opioid emergencies. A federal emergency could open up paths for those states to request federal grants for specific purposes. Maryland, for example, has a tool that tracks overdoses in real time so emergency responders can identify where particularly potent synthetic opioids may be on the streets. Federal money could potentially allow other states to follow suit.